using System;

using System.Globalization;

​

​

public class Test

{

    public static void Main()

    {

        float input = 3.14156F;

        string format = "00.000";

        var nfi = new NumberFormatInfo();

        nfi.NumberDecimalSeparator = ":";

        string output = input.ToString(format, nfi);

        Console.WriteLine(output);

    }

}